Light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), in particular, large area LEDs/OLEDs, are prone to shorts due to small particles contaminating the LED/OLED substrate and/or layers in case of imperfect cleaning and handling during production. Since, in practice, not all defects can be detected in the final production quality control, the occurrence of small shorts during operation may not always be avoided.
The detection of such shorts in the light-emitting area of e.g. an OLED is important, because they may result in a significant increase in the temperature at the locations of the defects (known also as “hot spot” effects). This is due to the fact that the power distribution, which is substantially evenly distributed across the light-emitting area during normal operation, may be concentrated at a very small area in case of a short. The local temperature at a hot spot can easily reach values well above 100 degrees Celsius, which can damage the OLED and/or can even be dangerous to a human being.
Prior art methods for short detection are based on monitoring the LED/OLED voltage as an indicator for the presence of a short. For example, if the LED/OLED forward voltage falls below a predefined voltage threshold for a nominal constant driving current, the LED/OLED may be considered to be defective. This detection is dependent on a characteristic absolute voltage threshold that is rather sensitive with respect to production tolerances (resp. LED/OLED “binning”) and the corresponding LED/OLED (forward) voltage variants resulting therefrom as well as the environment temperature.
US-2008/0231198 discloses a circuit for controlling a current through an LED. The circuit includes a regulator for providing the current to the LED, a voltage monitoring circuit for monitoring a voltage drop across the LED and for providing a voltage reading signal based on the voltage drop, and a data converter logic circuit coupled with the regulator and the voltage monitoring circuit. The data converter logic circuit is arranged to control the regulator to adjust the current based on the signal, as well as being operable to detect a short-circuit of the LED based on a sudden drop in voltage.